Fluorinated surfactants have been long since used in emulsion polymerization process for the manufacture of fluoropolymers.
Traditionally, perfluorocarboxylic acid derivatives have been employed in said processes; however, due to environmental concerns related to the use of such compounds, fluorosurfactants comprising an oxygen-containing side chain have attracted increasing attention as emulsifiers. In particular, perfluorooxycarboxylates of general formula: RfO—CF2CF2—O—CF2—COOX, wherein Rf is a perfluoro(oxy)radical and X is an alkali metal cation or an ammonium cation have been considered.
Such perfluorooxycarboxylates were first disclosed in U.S. Pat. No. 3,271,341 (E.I. DU PONT DE NEMOURS) Sep. 6, 1966. This document teaches a manufacturing method wherein tetrafluoroethylene epoxide is polymerized in the presence of a suitable free-radical forming compound (e.g. activated charcoal) at low temperature, resulting in polyethers having general formula: CF3CF2—O—(CF2CF2—O)n—CF2COF, with n being an integer from 0 to 10. Subsequent distillation affords various fractions differing from one another in their polymerization degree: corresponding acids or salts are obtained from the acyl fluoride derivatives by hydrolysis and, for salts, simultaneous or subsequent reaction with a base. Nevertheless, this process suffers from the disadvantage that a distribution of polyether surfactants is obtained, so that yields of single particular target compounds might be low and the separation Steps for isolating the same very burdensome.
Since then, a number of alternative methods for the manufacture of perfluorooxycarboxylates have been developed, for example those disclosed in EP 1164122 A (ASAHI GLASS CO LTD) Dec. 19, 2001, U.S. Pat. No. 7,053,237 (ASAHI GLASS CO LTD) Nov. 24, 2005, JP 2006321797 (ASAHI GLASS CO LTD) Nov. 30, 2006, EP 2058291 A (ASAHI GLASS CO LTD) May 13, 2009, WO 2007/140091 (3M INNOVATIVE PROPERTIES COMPANY) Dec. 6, 2007, WO 2010/003931 A (SOLVAY SOLEXIS S.P.A.) Jan. 14, 2010 and WO 2011/003575 (SOLVAY SOLEXIS S.P.A.) Jan. 13, 2011
Some of such methods involve the fluorination of alcohols comprising at least one C—H bond and an ethereal oxygen atom; however, hydrocarbons containing functional hydroxyl moieties are generally unstable under conditions of traditional fluorination processes. Under such conditions, it is generally understood that compounds having hydroxyl groups decompose, with simultaneous release of HF and COF2, and subsequent formation of corresponding non-functional perfluorocompounds having one less carbon atom than the starting hydroxyl-containing compound. Therefore, such alcohols must be protected before fluorination, for example by conversion into esters of perfluorinated carboxylic acids or as esters of fluoroformic acid. Decomposition problems can be even more serious when the starting alcohol comprises a moiety of formula —CH2OCH2—, which is easily cleaved in the presence of HF developed in the course of the reaction.
In particular, EP 1164122 A discloses a process for producing fluorinated compounds wherein a primary hydrogenated alcohol is first converted into the corresponding ester, generally a partially fluorinated ester, as obtained by reaction with a (per)fluorinated acyl fluoride, and then subjected to fluorination in liquid phase. The so-obtained perfluorinated ester can be then thermally cleaved or decomposed with suitable agents, to obtain perfluorinated acyl fluoride corresponding to the starting hydrogenated alcohol.
Similarly, U.S. Pat. No. 7,053,237 discloses a process for producing a fluorinated ester, wherein a primary hydrogenated alcohol is protected via transesterification and then subjected to fluorination in liquid phase.
However, the above described processes have the drawback that, in order to prevent decomposition of the reagents due to the reaction exothermicity, it may be necessary to operate under diluted concentrations both of fluorine and of the hydrogen-containing alcohol. Furthermore, to obtain a fully fluorinated product, a large excess of fluorine over the stoichiometrically required quantity, is needed. These conditions might negatively affect the reaction rate, yielding low productivity of the overall process.
Furthermore, as already mentioned, in order to reduce fluorine consumption, protection of the alcohol moiety as an ester is generally performed using suitable perfluorinated carboxylic acid derivatives, generally acyl fluorides, whose availability might be costly and induce further Steps for appropriate separation, recovery and reuse.
As an alternative, hydrogen-containing alcohols can be protected under the form of fluoroformates before being submitted to fluorination.
Thus, U.S. Pat. No. 3,900,372 (PHILLIPS PETROLEUM) Aug. 19, 1975 discloses a process for the production of perfluorinated organic compounds from hydrogen-containing alcohols. The process comprises protection of the hydroxyl moieties of the hydrogen-containing alcohol by reaction with carbonyl fluoride to yield corresponding hydrogen-containing fluoroformates. Said fluoroformates are then subjected to an electrochemical fluorination Step, and the resulting perfluorinated counterparts still possessing the fluoroformate functionality are subsequently cleaved by the action of fluoride ions under reacting conditions for yielding corresponding acyl fluorides. Further, it is known that perfluorinated fluoroformates can be converted into fluoroacyl fluorides with loss of carbonyl fluoride, easy to separate and recover.
However, electrochemical fluorination is a burdensome and energy-consuming procedure, which is generally less economically and industrially acceptable than fluorination with elemental fluorine. Furthermore, yields in electrochemical fluorination are known to be mostly moderate or even poor, especially if high molecular weight compounds have to be fluorinated.
Attempts to fluorinate with molecular fluorine certain fluoroformates were disclosed in GB 1226566 (MONTECATINI EDISON) Mar. 31, 1971; this document teaches a process for the preparation of certain perfluorinated polyethers wherein possible terminal groups of acidic nature, such as formate moiety, are eliminated. Conversion by severe heat treatment of a perfluorinated polyether having a fluoroformate terminal group into a fluoroacyl fluoride is also described.
According to WO 2011/003575, fluorinated compounds, in particular perfluorooxycarboxylates, can be obtained by:
A. converting an at least partially hydrogenated alcohol into corresponding at least partially hydrogenated fluoroformate compound;
B. reacting said at least partially hydrogenated fluoroformate compound with fluorine in the presence of at least one (per)haloolefin comprising at least one carbon-carbon double bond and having at least one fluorine or chlorine atom on either one of the carbon atoms of said double bond, to obtain a perfluorinated fluoroformate compound; andC. cleaving and hydrolysing the perfluorinated fluoroformate compound.
This method allows carrying out the fluorination Step under mild conditions, thereby achieving high yields and selectivity and complete fluorination of the starting alcohol without using large amounts of fluorine. In particular, this document teaches that, in the fluorination reaction, the reagents can be used even without solvents and that the temperature can be maintained in the range of −100 to +50° C., without observing decomposition of the reagents; in the examples, an acyl fluoride of a partially fluorinated cyclic ether which does not comprise a —CH2OCH2— moiety is submitted to fluorination at a temperature of at most 5° C. However, the Applicant has observed that, even in the presence of a (per)haloolefin, alcohols comprising —CH2OCH2— are still susceptible to degradation when the fluorination reaction is carried out at a lower temperature than 0° C.